TVE-E 20 002
Examensarbete 15 hp
Juli 2020
Evaluation of potential marine
current turbine sites in North
American waters
Independent Project in Electrical Engineering
Tim Andersson
Muhammad Arsal Akram
Carl-Henrik Carlnäs
Tiffany Salisbury
Teknisk- naturvetenskaplig fakultet UTH-enheten Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student
Abstract
Evaluation of potential marine current turbine sites in
North American waters
Tim Andersson, Muhammad Arsal Akram, Carl-Henrik Carlnäs, Tiffany Salisbury
Suitable locations for marine current power generation were scouted. The specific turbines considered in this project are vertical axis turbines and require an water velocity of 0.8 m/s to start and has a system efficiency of 20%. In the beginning of the project focus was directed towards areas along Florida's coastal line with high water velocities tapping into the Gulf Stream. Data found the velocities did not meet the water speed requirements. Following this observation, it was decided to discontinue further research in the Florida region and divert the attention towards waters in Alaska. There current velocities were found to be significantly higher. Because velocities vary over time marine current power is not relevant in Alaska, but rather the closely related technology tidal power. Two areas in Alaska distinguished themselves, Cook Inlet and Aleutian Islands.Potential power and annual energy extraction were estimated for turbine stations at each site. A battery energy storage system was implemented to counteract varying water velocities. The most promising site could steadily deliver 269 kW and an annual energy production of 2.44~GWh per turbine.
Acknowledgements
Foremost, we would like to express our sincere gratitude to our supervisors Johan Forslund and Christoffer Fjällstedt for their advice, feedback and continuous support throughout the project. We could not have wished for better supervisors and guidance.
A special thanks goes out to Johannes Hjalmarsson for his guidance with energy storage systems. We would also like to thank Mats Ekberg for guidance on writing a technical report and presenting a project.
Lastly we would like to acknowledge the information and data provided by the National Oceanic and Atmospheric Administration which was of great help during the project.
Acronyms
EU European Union
USA Unites States of America
NOAA National Oceanic and Atmospheric Administration MATLAB Matrix Laboratory
BESS Battery Energy Storage System CSV Comma-separated Values AC alternating current DC direct current ID Identifier GWh Gigawatt hours MWh Megawatt hours
Contents
List of acronyms
ii
1 Introduction
1
1.1 Background
. . .1
1.2 Purpose
. . .2
1.3 Goal
. . .2
1.4 Limitations
. . .2
1.5 Method
. . .2
2 Theory
3
2.1 Ocean Currents
. . .3
2.1.1 Tidal Currents . . . 32.2 Current Energy Conversion
. . .5
2.3 Turbine
. . .6
2.4 Battery Energy Storage System
. . .7
3 Procedure
8
3.1 Interesting Sites
. . .8
3.1.1 Data . . . 8
3.1.2 Current Velocities . . . 8
3.1.3 Depths . . . 9
3.2 Depth Range and Turbine Dimensions
. . .9
3.3 Power Calculation
. . .10
3.4 Energy Storage System and Energy Estimation
. . .11
4 Results
13
4.1 Water Velocities - Statistical Analysis
. . .13
4.1.1 Florida . . . 13
4.1.2 Alaska . . . 14
4.2 Velocity Variations with Depth
. . .16
4.3 Power and Energy Estimations
. . .18
4.3.1 Estimating Average Power and Deciding Battery Size . . . 18
4.3.2 Final Power and Energy Results . . . 23
5 Discussion
24
5.1 Florida
. . .24
5.2 Alaska
. . .24
6 Conclusions
25
1 Introduction
1.1 Background
The use of current power is not a new innovation, in fact the oldest current power we know of dates back to the 4th century A.D. The oldest type of current power known is the vertical water mill. It was used during the middle ages and had a great impact on mankind [1]. Since then a lot has happened. We have invented new technologies and much more efficient ways of getting energy from different currents and they are now not only limited to streams and rivers.
Today current power plants can be placed in the ocean and use oceanic currents. There are two main current power stations that are in use, horizontal and vertical axised. The big difference between these is how the turbine blades are positioned. The horizontal axis turbine looks like the typical wind power station but it is placed under water. The vertical axis turbine looks like the one in Figure 2.2.
Current power is a renewable power source that could be a significant energy source in parts of the world with the right geographical location. Population is increasing rapidly [2], resulting in an increased need for power, which will lead to an increased need for energy sources.
Meeting the goals set by the Paris Agreement requires decarbonizing the power sector, making sources such as coal and diesel less viable[3]. The European Union (EU) also has a goal to be the first carbon neutral continent by year 2050 [4]. Therefore, renewable energy sources are becoming more attractive alternatives as a main source of power. However, the electricity production from most renewable sources can not be regulated in the same way as e.g. diesel or nuclear power.
This is due to the fact that there is no guarantee for a constant power output with renewable energy sources. For example a wind power plant generates energy by using the wind as a ”fuel” for its generator. The problem is that wind is not constant. Sometimes it blows more, sometimes less and sometimes not at all. This results in variations of output power for the power plant. Electricity generation must simultaneously meet its supply and demand. If generated power varies, a need of energy storage system arises.
Putting these two facts together means that the adaptation of renewable energy is a challenge. We propose one solution is to diversify the sources of production and find efficient means to store the energy provided by these sources to balance the production and consumption.
One way of diversifying is with the relatively unexplored marine current power and tidal current stations. The main benefit of the technology is that ocean current velocities are predictable and do not vary a lot over time. Moreover ocean currents move large volumes of water and energy density is high, making it a promising renewable energy source. Great benefits of tidal current power production are that it is reliable, predictable and environmentally friendly [5]. Other factors include that the stations are positioned underwater, protecting them from storms. For example wind powered stations can’t be used in some places of the world, like Florida, because of big storms. Florida gets struck by hurricanes every year and wind power stations would break in these strong winds. This is were the marine current power stations come in to play. They don’t get as affected by storms if they are placed deep enough. This makes them very reliable no matter what the weather conditions on land may be.
land-1 INTRODUCTION
1.2 Purpose
The purpose of this project is to select interesting sites in the state of Florida and Alaska and estimate the power and energy production through marine and tidal currents.
1.3 Goal
The main goal of this project is to promote renewable energy production and marine current power generation as an viable alternative. By analysing the power and energy production in the chosen areas, the chances of marine or tidal current power would increase in the future to be considered as a significant energy source. Thus, leading towards a greener and more sustainable environment. This goal can be achieved by dividing the process into four parts:
1. Find the interesting sites with respect to water velocities and depths. 2. Estimate the power generation for chosen sites.
3. Study the power variations and propose an energy storage system needed to level out the power. 4. At last, estimate the energy over a year.
1.4 Limitations
Following are the limitations of this project:
• The environmental aspects such as sea life has not been considered during the selection of sites. • The economic aspects have not been taken into consideration during the estimations of turbine
dimensions and energy storage systems.
• Traffic of watercraft having vessel going deeper than 5 meters have been ignored for the sites, where the interesting depth range started from 5 meters under the surface.
• The size range of the Battery Energy Storage System (BESS) has been decided to be minimum 1 MWh and maximum 20 MWh.
1.5 Method
All the analysis and study has been done using the official site of National Oceanic and Atmospheric Administration (NOAA). The interesting sites have been chosen on their website and then, the data of the chosen sites was imported to the mathematical tool and software Matrix Laboratory (MATLAB). Here the data was leveraged in order to make power and energy estimations.
2 Theory
2.1 Ocean Currents
The World’s ocean waters are subject to a variety of forces. These forces are driven by mainly three factors; wind, thermohaline circulation, and gravity. The forces produce motion of the oceans water, called currents. These currents move over vast distances with great velocities.
Winds produce currents that are at or near the surface of the ocean. When the wind comes into contact with the ocean water, the water rises in the direction that the wind blows, creating currents and waves. Density differences in ocean water creates currents due to temperature and salinity variations in different parts of the ocean. This process, known as thermohaline circulation, occurs at both shallow and deep ocean levels [6]. The Gulf Stream is one such ocean current driven by wind and thermohaline circulation. Originating as the name suggest in the Gulf of Mexico. It moves 56 million cubic meters of water every second. Speed varies in the ranges of 1.5 to 2.5 meters per second. It does not vary significantly over time, but rather spatially. Considering the density of water this means there is huge amounts of kinetic energy in the Gulf Stream [7].
The last major factor that causes ocean currents is the gravitational and centrifugal force of the Earth. These forces create the phenomenona known as tidal currents. These tidal currents are explained as follows.
2.1.1 Tidal Currents
Tidal streams are currents created by periodic horizontal movements of water in the tidal wave. This phenomenon usually occurs in the ocean waters and repeats itself with great regularity over a period of 12 to 24 hours. A tidal cycle can be described with low tide and high tide, also known as flood and ebb. During the high tide the water gathers near the coast line and during the lower tide the water is pulled away [8].
Tidal arises due to the gravity and attractive force between the Moon, the Sun, the Earth and the movement of the respective celestial bodies. In the Earth-Moon system, the Earth and the Moon acts like a single system. The Earth is attracted by the gravity force to the Moon and moves in an almost circular orbit around the common center of mass under the balance between the lunar gravitational force and the outward centrifugal force. The magnitude of gravitational attraction exerted by the Moon on the Earth’s surface will not be the same in all points due to the different distances from the Moon.
The surface of the Earth directly facing the Moon is where the gravitational pull is the strongest. This pulling power exceeds the centrifugal force causing the Earth’s oceans to bulge outwards in the direction of the Moon, i.e. high tide. But there is also a tidal bulge on the opposite side, the furthest away from the Moon. Here the Moon’s gravitational pull is at its weakest but at the same time the centrifugal force acts similarly equal in all places on the Earth, i.e. away from the Moon [9]. See Figure 2.1.
2 THEORY
Figure 2.1: A representation of the forces exerted on the Earth (not to scale).
As mentioned, the Sun also causes tides but they are much smaller in magnitude as the Sun is about 389 times further away from the Earth compared the Moon [10]. Mathematically, it can be understood by the help of Newton’s Gravitational Law, where the distance between the bodies is, in square, inversely proportional to the gravitational force between them. [11].
2 THEORY
2.2 Current Energy Conversion
The generation of electricity through marine currents is achieved by extracting the kinetic energy of water. Turbines have blades mounted on a rotor connected axially to a generator. Mass flow of the water hits the blades and generates momentum, causing the axis to rotate. The generator uses the kinetic energy to create two coupled magnetic fields. Interaction between these two fields will create an alternating current on the stator side.
Connecting the generator to the power grid requires same phase sequence, phase angle, frequency and amplitude. The frequency of the generated electricity is determined by rotor speed and indirectly water velocity. Ensuring the right frequency and other characteristics can be done through first converting to direct current (DC) and then converting back to alternating current (AC) with desired characteristics. Main magnitude of interest is power. In order to derive a useful equation first the kinetic energy of water per cubic meter is considered:
Ek=
mv2
2 (1)
where m is the mass of water and v is velocity of the water. Power is energy per time unit. Therefore the mass flow of water through a cross-sectional area is instead considered. Both sides of Equation (1) are differentiated with respect to time. The mass flow is then inserted into Equation (1) and the power equation is arrived at:
˙ m = ρAv Ek dt = dm dt v2 2 ⇔ ˙Ek = ˙ mv2 2 =⇒ Pturbine = Cp· ρAv3 2 (2)
In above equation P denotes power extracted by the turbine, A is the swept area of the turbine, ρ is water density. Finally a power coefficient Cpis added. Cpis a function of the tip-speed ratio, which is
the tip speed of the blade divided by the wind speed. It closely relates to the Betz limit, a theoretical efficiency limit stating all kinetic energy can not be converted.
Equation (2) specifies the ratio of kinetic energy that can be converted by the turbine. The generator, power electronics and transmission line contributes with mechanical, electrical and magnetic losses. Therefore a degree of efficiency for the whole system is considered, ηsystem, the delivered power to the
power grid can then be seen as
P = ηsystem·
ρAv3
2 THEORY
2.3 Turbine
Several types of turbines for underwater are found today on the market. The turbine blades are categorized as either vertical axis or horizontal axis. They can either be suspended under a floating platform facing downwards or mounted on the seabed. The technology behind the marine current power is similar to wind power, but deviations between these are the difference in density of water versus air. Water has a higher density than air, resulting in, for the same speed, the kinetic energy in flowing water is higher than that of air. More energy per unit area of the turbine can therefore be produced but it also means that higher demands on the material strength of the turbine must be made [12].
The turbine considered in the project is a vertical axis H-rotor sea floor-mounted turbine. A threshold velocity of 0.8 m/s is needed in order to start the turbine and the nominal speed is 1.2 m/s[12]. In this project a constant degree of efficiency ηsystemis used. It has been calculated to 19% and is in this
project rounded up to ηsystem = 20%[13]. In order to ensure water forces are not too large the width
is set to be twice the height of the turbine. An illustration is shown below in Figure 2.2 to visualize the design.
Figure 2.2: Illustration of the turbine on site, mounted on the sea bed. At the bottom of the turbine is an encapsulated generator. The height of the blade determines the depth range it covers. Swept area is the diameter of the
2 THEORY
2.4 Battery Energy Storage System
Tidal phenomena causes periodical variations of power generated in the time domain. These variations are not wanted and may be compensated for by using a BESS(Battery Energy Storage System). BESS help to create a more reliable and flexible grid system by balancing the supply and demand. There are a number of different methods for energy storage such as batteries, hydrogen storage, flywheels, capacitors and supercapacitors.
Different types of batteries are used for energy storage, all with different characteristics. For example, lead-acid batteries, sodium-sulfur batteries, nickel-cadmium batteries, lithium-ion batteries, zinc-bromine batteries[14]. In this project no specific batteries were taken into account.
3 Procedure
3.1 Interesting Sites
The project began with the supervisors providing a reliable source, the official website of NOAA [15], an American government agency that provides tide and current predictions and historic data and other oceanographic and meteorological conditions.
Sorting of interesting sites in the big Gulf Stream, coasting the state of Florida, started with two parameters taken into consideration, velocity and depth. In order to get the turbine to operate and start generating electric power, a current speed of 0.8 m/s is required. Moreover, to place the turbines under the water, without it interrupting the large boat traffic, depths of the sites were examined. A world map displaying different types of measurement stations on NOAA’s website was used to find the most interesting sites. Apart from water velocities and site depths, these stations contain several different types of data such as tidal currents, air temperature, water temperature, water levels, winds and humidity.
During the early stage of the project, it was realized that the stations in Florida are unsuitable for the task (see section 4 and 5). Due to this, the project’s focus was shifted from Florida to Alaska. It shall be seen in the section 4 that, apart from Figure 4.1 and Table 4.1, all the results are from the areas of Alaska. This is because, as mentioned above, the project proceeded with Alaska instead of Florida. 3.1.1 Data
The map on NOAA’s website shows different types of stations that contain either historical or predicted tidal data. The stations called ”current” show historical measured data while the stations named ”harmonic” and ”subordinate” show tidal prediction data. Harmonic stations are stations with tidal datums and tidal harmonic constants [16], [17]. The subordinate stations are historic stations but they do not contain tidal harmonic constants.
Both historical and tidal predicted data have been used in this project. To overview the water velocities and sites’ depths, predicted data was used as it was available for all the stations. This overview helped to show if the area had any apparent potential for marine current power or not. Once this was completed, each site on the area was carefully studied and those sites that showed highest velocities and depth values, as well as had historical measured data available, were selected. After the sites were selected, historical velocity data of the sites was downloaded from NOAA’s website as Comma-separated Values (CSV)-files and imported into MATLAB for the power and energy estimations.
Further details on the specifics of velocity and depth used for the site selection are mentioned in the next two sections, i.e., 3.1.2 and 3.1.3
3.1.2 Current Velocities
Those stations which had current velocities above 0.8 m/s were chosen. The graph in the figure below (Figure 3.1) shows an example at station UNI1007 during the time period from 2010-06-21 to 2010-06-28. It can be seen in the graph that the velocity is above 0.8 m/s during large period of the time. Note the figure uses cm/s on the y-axis.
3 PROCEDURE
Figure 3.1: Velocites from the station Akun Strait, UNI1007, during one week [18].
3.1.3 Depths
The second criteria to select the interesting sites was the depth of the station. If the depth of any station was less than 10 meters, it was excluded. The reason behind this was that the turbine had to be placed below the surface to avoid interrupting the small boat traffic and the size of the generator under the turbine also needed to be taken into consideration. Thus, the ranges were decided to be at least 5-10 meters from the surface and 1-2 meters from the bottom to account for the generator and lowest point of the turbine.
3.2 Depth Range and Turbine Dimensions
After the stations were selected, an analysis of the velocity at different depths was performed. To do this, the data for each selected station was downloaded. Every station had multiple CSV-files, each corresponding to velocities for a certain depth and a time duration(usually a month). After downloading the data, the names of all the files were edited by putting their respective depths in their names (see step 8). Lastly, a MATLAB-script was written to perform the analysis. The purpose of the analysis was to determine how the cubic velocities are changing with depth. This was to help decide the depth range in which the turbine is to be placed.
Following are the steps, and their explanation, taken by MATLAB-script to help perform the above mentioned analysis:
1. The user is asked to enter the number of stations.
2. For each station, the user is asked to select a respective folder(station) and input the number of files (depths) present in the folder.
3 PROCEDURE
different time points. The time duration is one month and number of depths vary from station to station.
4. From the first column, the maximum value of velocity and its index is extracted. It is should be understood that index corresponds to a time point.
5. For the same index (time point), velocity values from rest of the columns are also extracted. 6. All these values are saved in an array and then cubic of these values is taken, as the main interest
is to analyse how the velocities are changing in cubic with the depths (see Equation 3).
7. The maximum value of cubic velocity is extracted from this array and all the velocity values are divided by it. This is done to normalize the cubic velocities.
8. When the program goes through each file, it also saves depth value of each file in a separate array.
9. The array of normalized cubic velocities and array of depths are then plotted with the velocity values on x-axis and depth values on y-axis.
10. This process is repeated for every station.
The above mentioned steps were executed for all the selected stations from two different areas of Alaska i.e., Cook Inlet and Aleutian Islands. To decide the depth range, an area on the graph was marked in between which most of the plots were achieving their maximums. The results can be seen in Figure 4.5 and Figure 4.4.
Once the depth range was decided, the dimensions of the turbine were decided accordingly. For example, if the depth range was chosen from 15 to 20 meters below surface, then the dimensions of the turbine would be 5 x 10 meters, where 5 m is the height of the turbine blades and 10 m is the diameter of the circular area they are covering. The turbines are chosen to be twice as wide as high in order for them to be robust enough to handle mechanical stresses occurring from potential differences of water velocities between the top and bottom part of the turbine blades.
3.3 Power Calculation
Once the dimensions of the turbine were decided, a MATLAB-script was written to calculate the power. This script was then used to estimate the electrical power that can be extracted from the selected sites. The results from these power calculations can be seen in Table 4.4. Following are the steps, and their explanantion, taken by MATLAB-script to calculate and plot the power:
1. The user is asked to select the folder that contains the velocity data files. The folder corresponds to a specific station.
2. The user is then asked to enter the number of files that are present in the selected folder. This number of files, in this case, corresponds to the number of depths, at which the velocity data has been measured. Each file has velocity data for one depth for a time duration of one month. 3. The user is asked to enter the dimensions of the turbine.
3 PROCEDURE
different time points. The time duration is one month and number of depths may vary from station to station.
5. All the velocities are converted from centimeter per second to meters per second and then the cubic of all the velocity values is taken.
6. An average value of each row of the matrix is taken. This gives the average cubic velocity values of different depths at each time point. It should be understood that after performing this step the matrix with several columns is now converted into a single column, array. This array, as mentioned before, contains all the average values of the cubic velocities with respect to depths for every single time point of measurement.
7. These average cubic values of the velocities and the saved value of turbine dimensions are then used as per Equation (3) with nsystem = 0.2to calculate and plot the power for each time point
(the power as a function of time).
8. It should be understood that the power calculated is stored in a array as a function of time, as it is calculated using the average cubic velocities’ array mentioned in the step 6.
9. A mean value of the power array is calculated. This value is the expected average power estimation that can be delivered to the grid.
3.4 Energy Storage System and Energy Estimation
The results from the power calculation displayed variations, as expected. To make the generated electrical power useful a BESS was implemented in the same MATLAB-script mentioned in section 3.3. This BESS was considered lossless. Apart from what is mentioned in the previous section, the MATLAB script takes the following steps to implement a BESS:
1. The user is asked to enter the number of times the program should run. The purpose of this is to perform manual iterations in order to decide the battery size and average estimated power delivered to the grid.
2. The power that shall be delivered to the grid, estimated average power, is decided, in first iteration,by setting it to mean power. In other iterations, the user is asked to input the estimated average power after analyzing the results from the first iteration.
3. Battery size is decided, in first iteration by setting it to maximum value of 20 MWh. In other iterations, the user is asked to input the battery size after analyzing the results from the first iteration.
4. For each point of the time, it is checked if the generated power is equal to, less than or greater than the estimated average power.
5. If the generated electrical power is greater than the estimated average power, the remaining power is stored as an energy in BESS. If the BESS is full, the power is assumed to be wasted. 6. If the generated electrical power is less than the estimated average power, the BESS discharges
3 PROCEDURE
8. If both generated electrical power and BESS, even together, can not deliver the estimated average power, the power delivered to the grid is considered null. In order words, it is considered to be a blackout.
To avoid the blackouts, mentioned in the step (8), the number of iterations is asked from the user in step (1). In the first iteration, the program runs with the estimated average power equal to the exact mean of the generated electrical power and the maximum BESS size i.e., 20 MWh. It plots the results and allows the user to analyse the plots. The user can then adjust the estimated average power or/and the BESS size in the next iteration to avoid any blackouts.
In the first iteration, as mentioned above, battery size is set to max. If blackouts are present in the results, the average estimated power was decreased by 10 kW. The power was decreased on every iteration until it was leveled out (no blackouts). After that, the battery size was decreased by 1.0 MWh on every iteration to find the minimum battery size to deliver the average estimated power to the grid. An example of this iteration process can be seen in the Table 4.3.
After the average estimated power with the minimum battery size, the energy per year was calculated by multiplying it by total hours in an year, i.e., 8760 hours. The same procedure was followed for each station and the results can be seen in Table 4.4.
4 Results
4.1 Water Velocities - Statistical Analysis
Data gathered through NOAA was processed in MATLAB and used to plot the boxplots. The lowest data point represents the minimum velocity, the highest data point represents the maximum velocity, the middle data point is the median and the “blue box” describes the velocities of the lower and upper quartile. The lower quartile is the middle value between the smallest number and the median of all data and the upper quartile is the middle value between the largest number and the median of all data. 4.1.1 Florida
Figure 4.1: Representation of the eight stations with high velocities in Florida. Green line displays the minimum velocity needed.
In Figure 4.1 above, measurement stations from Florida with high velocities were chosen along with a few additional stations to give a broader view of the current velocities. The boxplot displays one station with median velocities above 0.8 m/s. Table 4.1 below complements the boxplot by showing measure depth of the velocities. It contains names of the stations corresponding to their station IDs and the approximate depths of the stations.
4 RESULTS
Table 4.1: Stations in the Florida area which have been plotted in the bloxplot 4.1. Each station can make measurements at several depths and therefore both are displayed to show all measurements are not taken at the sea floor.
Station Name ID Measurement Depth [m] Station Depth [m] Blount Island East SJR9807 09.1 16.5 ICW Intersection jx0301 10.7 11.9 Miami Harbor Entrance MIH0901 10.7 13.7
Old Fernandina FEB1104 07.3 10.4
Old Port Tampa Bay t02010 09.7 12.2 Port Manatee Channel t03010 07.9 13.0 Fort Pierce Intel Entrance FPI0901 10.1 13.1 St.Johns River Entrance SJR9801 09.1 15.5
The data gather through measurement stations in the Florida region indicated the area does not have sufficiently high current velocities to meet the turbine requirements. Only one station in Figure 4.1 had a median velocity over the threshold value of 0.8 m/s. However the figure also shows the velocities vary greatly and are sometimes almost zero. Table 4.1 shows the site barely satisfy depth requirements. 4.1.2 Alaska
The results from the research done in Alaska can be seen below in the boxplots.
Figure 4.2: A boxplot representation of stations with high velocities in the area of Aleutian Islands, Alaska. Green line displays the minimum velocity
4 RESULTS
Figure 4.3: A boxplot representation of stations with high velocities in the area of Cook Inlet, Alaska. Green line displays the minimum velocity needed.
Boxplot 4.3 and 4.2 above outline sites in Alaska with high velocities. As seen in the figures all of the sites have median water velocities over the threshold value of 0.8 m/s.
Table 4.2: Table showing the names, station ID numbers, the depth of measurements and the depth of station
Station Name ID Measurement Depth [m] Station Depth [m] Aleutian Islands
Akun Strait UNI1007 14.3 30.9
Akutan Pass UNI1010 09.1 72.8
Avatanak Strait UNI1006 12.2 86.0
Baby Pass UNI1009 06.7 50.3
Derbin Strait UNI1022 10.1 77.8
Konets Head UNI1020 09.8 63.4
Udagak Strait UNI1012 05.8 42.5
Umnak Pass UNI1019 11.6 88.3
Cook Inlet
Knik Arm COI0301 03.0 21.4
Fire Island COI1209 01.8 24.1
Point Possession COI1207 05.8 52.0 North Forelands COI1204 03.0 28.1
4 RESULTS
The table shows that the measurement depths are sufficient i.e., greater than 10 meters. The stations lie at deeper depths than in Florida and the sites are deep enough for the depth requirement of 10 meters. An analysis on how the velocities in cubic vary with the depths was performed for each site from Table 4.2.
4.2 Velocity Variations with Depth
Figures 4.5 and 4.4 shows a representation of how the velocities in cubic varies with depth. The velocities are close enough to warrant using the average velocity over the depth range.
Figure 4.4: Displays variation cubic current velocities with depth. The velocities are normalized for easier comparison. The graph helps determine which depth range would be most beneficial for the turbines. The black box
4 RESULTS
Figure 4.5: Displays variation cubic current velocities with depth. The velocities are normalized for easier comparison. The graph helps determine which depth range would be most beneficial for the turbines. The black box
shows the chosen turbine depth range.
Note how the maximum cubic velocities tend to be reached in the range of 0-10 meters from the surface. Turbine size were chosen so that the blades covered the depth ranges for Aleutian Islands and Cook Inlet to be 10-20 meters and 5-10 meters respectively.
4 RESULTS
4.3 Power and Energy Estimations
In this section, it is shown how the average estimated power delivered to the grid and battery size was decided. An example from a station in Alaska has been chosen to represent the process followed to perform this task. The final results from all the selected stations are also represented in this section. 4.3.1 Estimating Average Power and Deciding Battery Size
Figure 4.6: Graph shows station UNI1007 power output plotted over a month. The energy in the battery is used to decide battery size and has an separate y-axis. The black line is the output to the power grid and desired to be held constant, but because the power variations are high, there are blackouts
present in the output power to the grid.
Figure 4.7: Graph shows station UNI1007 power output plotted over a month. The energy in the battery is used to decide battery size and has an separate
4 RESULTS
Figure 4.8: Graph shows station UNI1007 power output plotted over a month. The energy in the battery is used to decide battery size and has an separate y-axis. The black line is the output to the power grid and desired to be held constant. Delivered power to the grid is decreased from previous iteration
and a minor blackout can still be observed.
Figure 4.9: Graph shows station UNI1007 power output plotted over a month. The energy in the battery is used to decide battery size and has an separate y-axis. The black line is the output to the power grid and desired to be held
4 RESULTS
Figure 4.10: Graph shows station UNI1007 power output plotted over a month. The energy in the battery is used to decide battery size and has an separate y-axis. The black line is the output to the power grid and desired to be held constant. The battery size is decreased from previous figure and still
there are no blackouts.
Figure 4.11: Graph shows station UNI1007 power output plotted over a month. The energy in the battery is used to decide battery size and has an separate y-axis. The black line is the output to the power grid and desired to be held constant. The battery size is decreased from previous figure and no
4 RESULTS
Figure 4.12: Graph shows station UNI1007 power output plotted over a month. The energy in the battery is used to decide battery size and has an separate y-axis. The black line is the output to the power grid and desired to be held constant. The battery size is decreased from previous figure and
results in blackouts. Therefore iteration #6 is the final result. Table 4.3: Iteration process for average power estimation and minimum battery size for the station Akun Strait, Alaska. The green highlighted row
shows the decided result.
Iteration Battery Size [M W h] Power Delivered [kW h] Blackouts
1 20.0 299 Yes 2 20.0 289 Yes 3 20.0 279 Yes 4 20.0 269 No 5 19.0 269 No 6 18.0 269 No 7 17.0 269 Yes
Figures 4.6 to 4.12 represents an example of iteration process to decide average estimated power and battery size from a station called Akun Strait [UNI1007], Alaska.
• Power Generated: The blue graph represents the estimated power that is generated from the turbine.
• Mean Power: The red line represents the actual mean of the generated power.
• Power Delivered: The black line represents the average power that can be constantly delivered to the grid.
4 RESULTS
As it can be seen in Figure 4.6, 4.7 and 4.8, the power delivered to the grid has several blackouts despite charging the battery to maximum value, i.e., 20 MWh. But in the fourth iteration the blackouts have disappeared. That is the decided delivered power to the grid. Then to decide the minimum battery size, its size is decreased by 1 MWh. After decreasing the battery size, in figure 4.12, the blackouts appear again. That is why the results from figure 4.11, is the final result for the station Akun Strait. This result is mentioned in the table 4.4.
It is also worth mentioning that the blackouts that are present in the beginning of the graphs have been ignored. The reason behind this is that, the battery is not charged. If a charged battery is assumed from the beginning, these blackouts would not have been there.
Table 4.3, complements all the seven figures above. It shows how the delivered power to the grid, average estimated power, was adjusted and minimum battery size required was that achieved. Table 4.4, on the next page, present the results of power and energy estimations from the sites of Aleutian Islands and Cook Inlet respectively.
4 RESULTS
4.3.2 Final Power and Energy Results
Table 4.4: This table shows the station ID, the most interesting depth interval where the current is the strongest based on figure 4.2. Data available for depth range is based on what data was available from NOAA. Turbine dimension is the height of the blades times the diameter of the turbine(swept area, see Figure 2.2). Mean Power is the mean value of the power generated by one turbine with the set turbine blade dimensions. Average power delivered is the maximum power the given battery size can steadily deliver. Energy per year is the annual energy the system produce with one turbine and a battery.
Station ID Data Available For Depth Range [m] Turbine Dimensions [m2] Mean Power [kW ] Average Power Delivered [kW ] Battery Size [M W h] Energy Per Year [GW h] Aleutian Islands, Alaska - Interesting Depth Range 10-20 m
UNI1007 09-23 14x28 299 269 17 2.44 UNI1011 11-23 12x24 251 241 17 2.11 UNI1010 11-23 12x24 185 165 14 1.45 UNI1020 09-21 12x24 119 119 13 1.04 UNI1003 10-22 12x24 118 108 11 0.95 UNI1019 10-22 12x24 094 074 03 0.64 UNI1009 12-20 08x16 071 061 05 0.53 UNI1022 13-19 06x12 052 042 02 0.37 UNI1006 15-21 06x12 033 023 02 0.20 UNI1012 10-20 10x20 026 016 01 0.14
Cook Inlet, Alaska - Interesting Depth Range 05-10 m
COI0301 05-10 05x10 037 027 02 0.24 COI0508 05-11 06x12 032 022 02 0.19 COI1209 05-10 05x10 020 010 01 0.09 COI1207 05-09 04x08 016 006 01 0.05 COI1204 06-10 04x08 014 004 01 0.04 COI1210 05-09 04x08 014 004 01 0.04
5 Discussion
5.1 Florida
Data from Florida did not meet the requirements of the turbine. The measuring stations were located too close to the coast and not close enough to the main arteries of the Gulf Stream. This close to the coast the depth is too shallow. Furthermore current velocities are under the threshold velocity a substantial amount of the time, only the peaks generating power. Even though data of current velocities in the Gulf Stream were unavailable, it may still be a good location for marine current power.
5.2 Alaska
Several sites from Alaska are potential turbine sites. The sites producing the most annual energy in Table 4.4 are in Aleutian Islands. The three best sites are UNI11007 producing 2.44 GWh, UNI1011 producing 2.11 GWh and UNI1010 producing 1.45 GWh annually. In Cook Inlet COI0301 produces the most at 0.24 GWh. These results are for one turbine. Alaska annual energy consumption is 6.4 TWh[19]. This means at the most promising site, UNI1007, 1000 turbines could provide 38% of the annual consumption. Roughly 2600 turbines would be required to deliver 100% of the Alaskan energy consumption. The areas of the sites are huge and the biggest limiting factor would be installation costs.
One major characteristic of hydropower is how it relates to the cubic velocity, as shown in Equation (2). This means power generated mainly depends on velocity and great emphasize are put on having high velocities. Small variations are amplified and lead to large differences in output power. This is both a strength and a weakness. Figure 4.6, 4.7, 4.8, and 4.9 shows this phenomena. Tidal currents vary over the day, causing the power output to greatly vary. The BESS implemented to stabilize the power delivered to the power grid have to be very large to compensate.
Simplifications and assumptions made may result in lower power generation. The power coefficient is not in reality constant. It varies with speed, and nominal current velocities of the turbine are lower than the ones seen in Alaska during peak hours, leading to lower efficiency. The error of these simplifications related to power generation are only significant at small velocities as power is dominantly determined by velocity. Because the turbine cannot generate power below threshold values this error is even less significant.
Other simplifications are prone to larger errors. All the power and energy values mentioned in Table 4.4, are rounded off to the closest integer value. Most importantly the BESS was considered lossless. Even without losses all sites with annual energy generation over 1 GWh has battery sizes over 10 MWh. This implicate huge installation costs and the introduction of non-ideal batteries results in larger battery sizes.
Velocities vary with depth as seen in Figures 4.4 and 4.5. The figures show maximum velocities are reached near the surface. The sea bed-mounted nature of the turbines considered may therefore not utilize the site to its maximum capacity.
Each station were deployed on average about a month and the data gathered during this period is extrapolated to estimate annual energy production. Seasonal variations of velocities exists and therefore introduce an error to the estimation.
6 Conclusions
The results from Florida indicate that this coast line is not suitable for the envisioned type of marine current energy converter with regards to the velocity of the currents. Further, it can not be concluded if the Gulf Stream farther away from coastline has the potential for marine current power or not. This is due to the fact that NOAA has no measurement data from locations close to the Gulf Stream. The estimated power from each site shows tidal power may be a viable option of electricity production in Alaska.
One main concern identified was that the produced power varied a lot and very large batteries were needed to deliver constant power to the power grid. If ten turbines are placed instead of one per site then ten large batteries are also needed to be placed. Price of the batteries will scale linearly as they offer no scaling benefits.
Summarizing, if the large batteries can be provided to each turbine tidal current power is an alternative power source in Alaska.
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